Coaxial transmission line filter system



June 1 v. H. RUMSEY' ETAL 2,557,567

COAXIAL TRANSMISSION LINE FILTER SYSTEM Filed March 19, 1946 2 Sheets-Sheet 1 I I 17'; E

VICTOR H. RUMSEY PERCY G. SALMON June 19, 1951 v. H. RUMSEY ETAL 2,557,557

' coAxIAL TRANSMISSION LINE FILTER SYSTEM Filed March 19, 1946 2 Sheets-Sheet 2 20- WW G -i I? Q ME I I o HHHHH HHHHH VICTOR -H.- RUMSEY PERCY G. SALMON Patented June 19, 1951 COAXIAL TRANSMISSION LINE FILTER SYSTEM Victor H. Rumsey and Percy G. Salmon, Washington, D. 0., assignors to Minister of Supply -in His Majestys Government of the United Kingdom of Great Britain and Northern Ireland, London, England Application March 19, 1946, Serial No. 655,626

6 Claims.

. i This invention relates to electrical frequency selective devices and in particular to frequency selective filter devices for use in a' coaxial line system in which the filter device is wholly contained in the-coaxial line.

In numerous applications involving a coaxial line connection between two points it is necessary to interpose a frequency selective device at some point in the system. Possible locations for the frequency selective device are at either end of the line or at some intermediate point along the line. In numerous installations it may occur that, due to space limitations, the location of such a device at either end of the line is not practical. In such a case an intermediate mounting position is required.

- It is therefore an object of the present inven-j tion to provide a transmission line filter system capable of producing a maximum attenuation of one frequency band and minimum attenuation of a second frequency band.

Another object of this invention is to provide a filter device for use in a coaxial line system,

said device being wholly contained in the coaxial line.

Another object of this invention is to provide a filter device for insertion in acoaxial line, said filter device providing maximum attenuation of one band of frequencies with minimum attenuation of a second frequency band.

Other and further objects and features of the present invention will become apparent upon a careful consideration of the following detailed description when taken together with the accompanying drawing, the figures of which illustrate typical embodiments of the invention and the mannerin which those embodiments may be considered to operate.

In the drawing,

Fig. 1 shows in longitudinal section the typical embodiment of the features of the invention.

Fig. 2 shows, also in longitudinal section, a second embodiment of the invention.

Figs. 3 and 4 show electrical equivalents of the apparatus of Figs. 1 and 2. V

Fig. 5 is a plot of attenuation versus frequency in the vicinity of the reject band of the filters of Fig. 1 and Fig. 2.

According to the general concept of the present invention, maximum attenuation of a signal of one frequency contained in a coaxial transmission line with a minimum attenuation of another frequency is achieved through the placement in one conductor'thereof-of two shorted coaxial line sections separated by an impedance converting section of coaxial transmission line.

For optimum attenuation of energy at a par-.--

Also incorporated into the devices subsequently to be described in detail are certain broad banding features which widen the attenuation band and the transmission band rather than restrict them to specific frequencies, as described above.

Referring now to'Fig. 1, a coaxial transmission line filter system is shown comprising an outer conductor III and an inner conductor IIIA having hollow ends I I and I2. Extending into each of these ends and to a depth approximately equal to a quarter wavelength of the frequency ofenergy to be attenuated is an annular cavity as identified by the numerals I3, I I. Connected to the inner conductor at the bottom of each annular cavity is a conductive member possessing an enlarged central portion I5. This enlarged central portion is also approximately a quarter of a wavelength in length at the frequency to be suppressed. Support for the inner conductor assembly within the outer conductor I0 is provided by an annular ring of insulating material I6 inserted between the enlarged conductive portion I5 and the outer conductor III.

For optimum attenuation of a particular frequency, the depth of the annular cavities I3, I4 and the length of the enlarged portion I5, are each equal to a quarter of an electrical wavelength at that frequency. Thus, cavities I3 and M will appear as shown in Fig. 3 as parallel resonant, highly resistive paths M, I8 interposed serially in a first conductor I9 of an equivalent two wire transmission line. The raised portion I5 will appear as a pi-section' filter 2n interposed in the first conductor I9 between elements I1, 18- and connected to the second wire 22 of the transmission line. 1

To produce a minimum attenuation of energy at a second, for example, lower, frequency it is desirable that the entire assembly including the annular cavities I3, I4 and the enlarged central portion l present a matched impedance to the transmission line at that frequency. To fulfill this requirement, it is necessary that the impedance looking to the left direction (in Fig. 1) from the open end H beequal to;the impedance also looking in the right direction (in Fig. 1) from the open end I I.

To fulfill this matched impedance requirement at the exemplary lower frequency, transmission of which without attenuation is desired, it is necessary that the enlarged section l5 function as an impedance converting .tr'ansmis'sion line section (24 in Fig. 4) changing the inductive impedance 23 introduced by the annular ,cavity H) to the conjugate of the inductive impedance 24 introduced by the annular cavity l4. Since the length of the transmission .line,se.ction .24 is fixed by the quarter wavelength requirement of the attenuation frequency, the only method of altering the impedance converting ability of section 24 is by adjusting the characteristic irripedance of the section until the proper impedance conversion is realized. The characteristic impedance (Zc) of the section 24 is readily calculated by the formulas:

where:

Zc=required characteristic impedance of line sec- -,tion24 L=length of the enlarged section I5 (24) x=wavelength of the. pass frequency R=characteristic impedance of line comprising conductors in and IOA X=reactance of I each of theannular sections 13 and I4 given by a'X in subsequent equation Equation II jX =jA tan where: ximpedance of annular sections 13, A= characteristic impedance of the annular coaxial sectionsl3, I4

L length of the min lar sections l3, M

Such a large value of (A') is notcohsistent with the requirement of minimum attenuation of the pass frequency which would require small values of (A) in the same equation, for example, where (L) is appreciably less than 4 at a lower frequency. A working compromise with a value of (A) in sections l3, M in the range Of 40-50 ohms was found to give the best results.

Thus far the discussion has been limited to conditions of attenuation of one frequency and the transmission of another frequency bythe filter system. Frequently broad band action in the vicinity of one or both frequencies is required. To achieve this action, further compromises must be made inthe filter design.

, In one method of broad banding typically employed where a band width of 20% is required around both thepass and the reject frequencies, the resonant cavities I3, I4 were made of slightly different depths with one cavity resonating at a frequency. (f below the central frequency of the reject band and the other resonating at a frequency (f2) above, the central frequency. This produced a dual peak rejection characteristic as shown in Fig. 5, which is a plot of attenuation versus frequency with both coordinates increasing in thedir'e'ctions as shown; I N

The best overall rejection over a frequency band from (fa) to (fb) results when the rejec tion at frequencies (fa), (fbland (f0) is equal: Calculation of (f0) is by equation (fc =fafb). To

achieve this effect, the frequencies (f1, is) can be calculated from the following equations:

Equation III f1=0.854fa+0.147fb Equation IV f2=0.147fa+0.854fb where all symbols are as previously set forth.

This compromise upsets somewhat th match ing previously obtained in the pass band because the difference in the lengths (Li) and (L2) of the cavities I3, [4 does not permit the employment of the simple Equation I. Equation I may still be employed, however, provided that the characteristic impedances (A1) and (A2) of cavities l3, l4, respectively, are selected so that (A, tan equals (A tan at the central frequency of the pass band and that the value of (L) as used in Equation I is the electrical leng'thof the impedance converting line section [5. For this reason the diameter of the small left end section (Fig. 1) of the conductive member 15 is different from that of the small right end section (Fig; 1) For optimum operation it is desirable that the end effect due to the separation between the end ll of conductor .IOA and the left end (in Fig. 1) of the enlarged portion I5 be small. Also, for the same reason, it is desirable that the end effect due to the separation of end l2 of conductor HA and the right end (in Fig. 1) of the enlarged portion l5 be small. It is therefore desirable that these openings be no larger than is required to prevent voltage breakdown thereacross. V

An alternate filter arrangementis shown in longitudinal section in Fig. 2. This system is similar in many respects to the filter of Fig. 1, however, certain features such as reduced overall length are advantageous where the available length of coaxial transmission lineis limited.

The filter of Fig. 2 is contained within an outer conductor 26 of a coaxial transmission line. A discontinuous inner conductor having ends 2l, 28 is contained withinconductor 26. Mechani cal connection of the ends 2' and is provided by means of an axially disposed conductive rod 29 of somewhat smaller diameter. Surrounding rod 29 is a conductive cylinder 30, conductively attached to the end 21 of the inner coaxial conductor. A second, larger, conductive cylinder 3| is placed around cylinder 30. Cylinders 30 and 3| are attached, at their right ends (as shown in Fig; 2) to a short conductive cylinder 32. The connection of cylinders 30 and 3| to cylinder 32 may be made by conventional methods, as by welding or brazing.

Support of the inner conductor assembly within the outer conductor 26 is provided by means of an annular sleeve of insulating material 33 surrounding cylinder 3|.

It can'be seen that the filter of Fig. 2 is similar to that of Fig. 1. The cavity enclosed between the outer surface of cylinder 30 and the innerv surface of cylinder 3| corresponds to the shorted annular cavity l3. Similarly the line section comprising the outer surface of cylinder 3| and the inner surface of the outer conductor 26 corresponds to the section comprising the surface of the enlarged inner conductive member l5 and the inner surface of the outer conductor IU of Fig. 1. Also the annular cavity formed between the inner surface of cylinder 30 and the surface of rod 29 corresponds to the annular cavity I4. Therefore, the same design principles and formulae as applied to the construction of filters according to Fig. 1 are applicable to the design of the filters of the type of Fig. 2. The annular cavity formed between cylinder 30 and rod 29 is of a different length than that formed between cylinders 3| and 30 so that broad band action as described above is achieved.

It is preferable that the opening existing between the end of the outer surface of conductor 21 and the left end (as shown in Fig. 2) of cylinder 3| and also the opening between the end of conductor 28 and the cylinder 32 be small, typically no larger than is required to present voltage breakdown thereacross.

Typical dimensions employed in the filter.

shown in Fig. 1 where it Was desired to permit a passage of all frequencies from 950-1150 mc. and reject all frequencies having a wavelength from 8.5 to 10.5 centimeters are as follows, with dimensions in inches:

Inside diameter of ends of |A 0.3125

Inside diameter of conductor |0 0.875 Length of insulating material l6 0.622 Type of insulating material I6 poly-F Spacing between ends l2 and enlargement 0.1875

Typical dimensions (in inches) of filter of Fig. 2 for a reject band of 10.5-12.5 centimeters and for pass action as above.

Inside diameter of outer conductor 26 0.875 Diameter of inner conductor 21, 28 0.375 Diameter of rod 29 0.053 Inside diameter of cylinder 30 0.125 Outside diameter of cylinder 30 0.187

Inside diameter of cylinder 3| 0.4375

I in the appended claims.

From the foregoing discussion it is apparent that considerable modification of the features of this invention are possible, and, while the devices shown and described and the forms of apparatus for the operation thereof, constitute preferred embodiments of the invention, it is to be understood that the invention is not limited to these precise devices and forms of apparatus and that changes may be made therein without departing from the scope of the invention which is defined What is claimed is: g

1. A coaxial transmission line filter, for sup pressing energy of one frequency and transmitting energy of another frequency, comprising an outer conductive member and an inner conductive member, said inner conductive member comprising a plurality of quarter wave length discontinuities each being of an odd number quarter wave lengths long at the frequency of the energy to be suppressed, and an impedance matching section connecting said discontinuities in series operative to match the impedance of the filter section to the line at the frequency of the energy to be transmitted, said impedance matching section comprising a diametrically enlarged odd quarter wave length portion at the frequency of the energy to be suppressed formed in said inner conductor in series relation with said discontinuities.

2. A coaxial transmission line filter for suppressing energy of one frequency and transmitting energy of another frequency, comprising an outer conductor and an inner conductor, said inner conductor being provided with a plurality of odd quarter wave reentrant sections at the frequency of the energy to be suppressed formed in series therein and an impedance matchingsection connecting said reentrant sections in series operative to match the impedance of the filter section to the line at the frequency of the energy to be transmitted, said impedance matching section comprising a diametrically enlarged odd quarter wave length portion at the frequency of the energy to be suppressed formed in said inner conductor in series relation with said discontinuities.

3. A broad banded filter, for suppressing energy of one frequency and transmitting energy of another frequency, comprising; a coaxial transmission line having an outer conductor and an inner conductor, said inner conductor comprising at least a pair of spaced reentrant discontinuities therein each having a length of a quarter wave at the frequency of the energy to be suppressed and a conductive member bearing a raised central section having a length of approximately a quarter wave of the energy to be suppressed centrally positioned in said inner conductor between said reentrant discontinuities, said reentrant discontinuities forming two parallel resonant high impedance paths at the frequency of energy to be suppressed serially interposed in said inner conductor, said raised central section forming a shunt capacitance at the fre quency ofenergy to be suppressed, said raised central s'ection further forming with said outer conductor a section ofimpedance converting transmission line, whereby'irnp'edance introduced by the first reentrant discontinuity at the frequency of energy of which transmission is desired, is cancelled by the impedance introduced by the second reentrant discontinuity at frequency of transmission.

4'. In a broad banded filter as defined in claim 3, an annular cylinder ofinsulating material placed around the raised central portion of said conductive member for positioning said member in the outer conductor of the coaxial transmission line.

51A coaxial transmission line filter for suppressing energy of one frequency and transmitting energy of another frequency comprising an outer conductive member and an inner conductive member, said inner conductive member comprising a plurality of tandemly connected line sections each having a length approximately a quarter wave length of the frequency of the energy to be suppressed, one of said sections further forming with said outer conductor a section of impedance converting transmission line for matching the impedance of the filter section to the line at the frequency of energy to be transmitted.

6. A broad band coaxial transmission line filter for supressing energy of one frequency and transmitting energy of another frequency, comprising; an outer conductor, an inner conductive member, said inner conductive member having an interruption therein with the ends of said interruption spaced apart a distance somewhat larger than a quarter of a wave length of the frequency of energy to be suppressed, a second conductive member having a diameter smaller than said inner conductor, coaxially and conductively connecting the ends of said interruption of said inner conductive member, a first conductive cylinder, in length less than said second conductive member and having. a first and sec ond end, concentrically placed around said second conductive member and conductively attached.

at a first end of said interrupted inner conductive member and separated from the second end of said interruption of said inner conductive member, said first conductive cylinder forming with said second'conductive member a first parallel resonant high impedance path at the frequency of energy to be suppressed, a second cylindrical conductive member concentrically placed around said first conductive cylinder and having a length less than said first conductive cylinder, conductively connected to said first conductive cylinder at the second end thereof, said second cylindrical conductive member forming with said first conductive cylinder a second parallel resonant high impedance path in series connection with said first parallel resonant high impedance path at the frequency of energy to be suppressed, said second cylindrical conductive member further forming with said outer conductor an impedance matching section operative to match the impedance of the filter section to the line at the frequency of the energy to be transmitted, and an annular cylinder of insulated material placed around said second cylinder for positioning said cylinder within the outer conductor of the coaxial transmission line.

VICTOR H. RUMSEY. PERCY G. SALMON.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Hansen et a1 Apr. 6, 1948 

